Stage lights often allow different kinds of features and effects to be projected onto a stage a typical stage light might be a pan and tilt controllable device, which is remotely controllable over a format such as DMX, and produces a beam with an output intensity of at least 150 W, but more preferably between 400 and 800 W.
Many such devices also allow very sophisticated effects, such as gobos, coloration, blurring, and other similar effects. Many of these effects may depend on whether the item used to adjust the light control is in or out of focus at a specific location.
Most stage lights have only a single focus location.